


Primal

by Sonamyluffer101



Series: Commissions [4]
Category: Undertale (Video Game)
Genre: Alternate Universe - Horrortale (Undertale), Animal Instincts, Animalistic Sans (Undertale), Bara Sans (Undertale), Biting, Breeding, Dom Sans (Undertale), F/M, Horrortale Papyrus (Undertale), Horrortale Sans (Undertale), Knotting, Marking, Mating Cycles/In Heat, Overstimulation, Sans - Freeform, Size Difference, Size Kink
Language: English
Status: Completed
Published: 2019-06-30
Updated: 2019-06-30
Packaged: 2020-05-31 12:34:16
Rating: Explicit
Warnings: No Archive Warnings Apply
Chapters: 1
Words: 3,081
Publisher: archiveofourown.org
Story URL: https://archiveofourown.org/works/19426087
Author URL: https://archiveofourown.org/users/Sonamyluffer101/pseuds/Sonamyluffer101
Summary: You hadn't seen Sans for a few days, so you decided to pop over and see how he was doing....You probably shouldn't have ignored the note on the door.___Horrortale Sans x Reader. Post-surface.





	Primal

**Author's Note:**

  * For [Landy_Ano](https://archiveofourown.org/users/Landy_Ano/gifts).



> This is another commission for [Landy_Ano!](https://archiveofourown.org/users/Landy_Ano/pseuds/Landy_Ano) I hope you enjoy it!
> 
> I take commissions through my discord server, which is down below in the end notes <3

You really did enjoy your job, but it did sometimes get tiring.

By the end of the day, you were always exhausted and on the verge of passing out. A smart person would go home and fall right asleep, but you, were not a smart person. You never went straight to your house. Instead, you were going to your boyfriends house to waste time and not get a proper night's sleep.

You didn’t mind, though. You hadn’t seen him in a few days since you had been working overtime every night for the past week. You weren’t complaining, you just really missed him. You walked up to his front door, and were about to open it when you noticed there was a piece of paper taped to the front of it. You could tell right away that the note had been written by Papyrus, due to the messy letters and use of capitals.

You removed the note from the door, careful not to rip the tape, and read it in your head.

‘HI HUMANS (SMALL FRIENDLY HUMAN AND POSTAGE HUMAN INCLUDED), PLEASE DO NOT GO INDOORS. MY BROTHER IS VERY MOODY AND WILL PROBABLY KILL YOU! THANK YOU AND HAVE A NICE DAY!’

...You weren’t exactly sure what any of that meant.

Why would Sans be moody and murdery? That sounded ridiculous and untrue… almost like an excuse so nobody would come over. You assumed you were ‘small friendly human’, but… maybe you were mistaken? Sans would always be happy to see you. He said that himself… it had to be some kind of misunderstanding. He would be disappointed if you didn’t go inside, right? He didn’t exactly  _ ‘invite’  _ you, but still. It had been a while, and he would be happy to see you regardless of his mood.

You put the note back on the door so nobody else would come in, and then you went inside.

You weren’t surprised to find that Sans wasn’t downstairs. He spent a lot of time in his room, regardless of his mood, so that was where you always found him. You were planning to go up to his room, but you went into the kitchen first to grab a quick snack. They never minded when you ate their food, hell, they encouraged it. It helped them feel like they were taking care of you in a way. 

You opened their fridge, which was stocked with food as usual, and took out an apple. You didn’t have a  _ huge _ appetite, but you could definitely use a snack. You weren’t surprised to see premade spaghetti in the fridge.

You hoped he wasn’t feeling sick.

You took your apple and started up the stairs, taking a bite of the fruit in your hand. It tasted sweet. It made you feel a little better just by taste alone. 

You stopped outside of Sans’s bedroom and knocked twice. “Sans?”

You would usually just go inside… but you felt like you had to knock with the note you read. You were worried about him, and worried something might be going on that you didn’t know about. You hated when they kept things from you, both Sans and Papyrus.

There was a small ‘thump’ that came from inside… you assumed he dropped something. Maybe you scared him.

“...Sans?” You said his name again, hoping you would get a proper response.

There was nothing for a moment, and you were about to go in, but…

There was a sudden slam on the door from the other side.

You screamed as you fell on your ass, confused and frightened. The slamming continued, and his arm broke through the wood, a harsh snapping sound echoing as it splintered and he reached around. You hadn’t noticed there was a lock outside of the door until he unlocked it… and you realized you  _ really _ should’ve listened to the warning.

But it was too late now. He knew you were here, and you had to face the consequences. You walked towards his room as the door opened, and you peered inside… it was dark, and you couldn’t see anything.

Until you were yanked inside by Sans.

You let out a small scream, shocked by how forceful he was. You began to struggle… he would never hurt you, right? He couldn’t… he wouldn’t.

“S-Sans, please!” You begged, fighting his grip. He didn’t speak.

He  _ growled _ at you.

You were shocked… he had never done that before. You couldn’t believe it. Were you about to die?   
  


You were carried over to his bed, and pushed down onto it. He crawled over you and stared down at you, his one eye light wide and crazy. You were kind of scared… but you knew that eye light cared for you.

...He wouldn’t hurt you.

You noticed he was mostly naked… only wearing his boxers, not even a pair of socks. It was… weird, since he usually wore a jacket ninety percent of the time. You were the only person who ever actually got to see him like this… you were lucky, really.

He peered into your eyes, as if he were looking for something. He didn’t seem to find it, though… so he looked everywhere else too. Your entire body, before he landed on your hand. You hadn’t even realized you were still holding the apple from before. You blinked as he picked up the apple and put it against your lips. You weren’t… you didn’t understand? You raised an eyebrow as you looked up at him, but he just… pushed it against your mouth again.

You took a bite, assuming that was what he wanted, and he seemed to relax. You took the apple from him, and he didn’t seem to mind, so you just… continued to eat it. You were hungry anyways.

You finished it, and were about to toss the core into the waste basket but he grabbed it before you could and ate it whole.

You couldn’t help but giggle at that one. “Ew.”

There was a low rumble in his rib cage, something too caring to be a growl, but too deep to be friendly and calming. It felt like you were being pressed against a speaker… the rumble making you vibrate slightly.

You felt your face turning pink. “...Calm down.”

He didn’t say anything, like you expected. He leaned down to your shoulder and gave it a bite. You were so used to him marking you at this point, that it didn’t even bother you. You just wrapped your arms around his neck in a lazy hug and sighed, waiting for whatever this was to pass.

...Then you felt him grind against you.

You gasped softly, and let go of him. 

“...What’s going on?” You didn’t understand… it was scaring you. He hadn’t even  _ said _ anything to you.

“let’s fuck.”

…

...Well, at least he was speaking.

“...O-Okay.” You wouldn’t mind that.

He didn’t say anything else. He just pulled his cock out of his boxers, already hard, before shoving your pants off so roughly they almost teared. These were your work pants… you couldn’t afford that.

“H-Hey!”

He had never been so rough with you before… he was always gentle and slow, probably afraid of hurting or scaring you, but it was like all of those fears and cautions were thrown out the window now.

“stay still,” He commanded, his voice deep and his words slurred. He sounded drunk… maybe he had been drinking? But no… you would’ve smelled alcohol on him, and there would have been empty bottles laying around. You were probably just overthinking. Maybe it was something you just… couldn’t understand.

He moved down and licked your clit, just once, making you shiver pretty violently… your whole body felt so needy all of a sudden, and you felt so sensitive. It was so strange. You watched as his eyelight contracted, and he had to take a couple of deep breaths… you didn’t understand. Did you do something wrong…?

He moved back up to hover over you again, going slow. You didn’t know what to do or what to say… you were afraid of making this worse.

“d-don’t... don’t move. if you move i might snap…”

…”...Snap?” You really didn’t understand.

“i won’t listen when you say stop.”

...Well that didn’t sound very nice.

You gasped when you felt one of his fingers slide inside of you, but you tried not to move too much. You just gripped onto his arm, needing something to hold onto, and took deep breaths. You knew he was big… you knew prep was always necessary.

“...good girl,” He praised, slowly moving his finger inside of you, curling it and thrusting it gently. You couldn’t help but moan, the feeling so familiar and wonderful. It had been over a week since you had last been like this… submissive and weak beneath him.

You missed it.

He used his other arm to pull your shirt up, exposing your breasts. You had always had such sensitive nipples… you wanted to reach up and pull your shirt back down out of embarrassment, and the cold air, but… you resisted. You knew now wasn’t the time to play shy, and you really didn’t want to get him even more worked up.

He leaned down and licked your right nipple, his dark blue tongue so pretty against your skin. You couldn’t help but moan as he continued to pump his finger inside of you, while also giving your nipple a lot of attention. Your grip on his arm tightened as he added another finger, and you moaned his name softly.

You felt so weak.

“S-Sans…” His name came out of your mouth like a worship… a prayer. You loved him so much, and he was the only one who could make you feel like this.

He shuddered, violently… he looked so insane with that grin plastered on his face.

“oh, baby girl… i’m gonna destroy you.”

You felt your entire face heat up, bright red. He was never the type to talk dirty… you were shocked by his words, and his sudden confidence. You weren’t complaining, not at all, but… it was definitely different.

And it seemed to be enough to push you over the edge.

You came around his fingers, a third one had been shoved in… you hadn’t noticed. You just mewled as you rode the wave and let your orgasm pass. He gently removed all three fingers, and licked them. It was so lewd… so arousing. It made you giggle with anxiety and excitement.

He didn’t laugh or smile… you could tell he was really into this now.

You looked down and watched as he began to position himself at your entrance, and then slowly started to push inside. You were okay for a moment, but… you quickly realized he was bigger than usual. You didn’t understand how it was possible, but Jesus… you felt so stretched already.

“S-Sans,” You said his name softly, anxiety filling you up a bit. Something just… felt weird about this.

He pulled your legs up and began to move further inside of you… there was no pain, just stretch and anxieties. 

“S-Sans, hold on,” You said softly, moaning softly, your voice shaky and nervous. It just felt  _ different _ somehow.

He kissed you softly, shutting you up momentarily. “shh... you can take it.”

You were so sure if you could… you trusted him, but you knew how big he was usually, and that was enough to stretch you a good amount, so this was a bit concerning. You didn’t wanna overdo it and hurt your body, but… you knew inside that Sans would never hurt you, especially not when it came to sex. He was very anxious about it, even more so than you were, so if he felt like you were fine, you  _ were _ fine.

“...Wh-Why are you acting like this?” You still didn’t understand his behaviour.

“heat,” He grunted.

“...Heat?”

He began to thrust, and you gasped, beginning to struggle. Your anxiety was getting the better of you, and you had forgotten his warning… so he pinned you down to the bed, growling, his anger terrifying you momentarily.

You froze, stiffening in fear. You wished you were less squirmy when it came to sex… then this wouldn’t be so hard to deal with. You had issues with staying still as it was.

But what did he mean by heat?

You moaned as he bucked into you, acting like a wild animal. You weren’t used to rough sex, especially with somebody his size. Your body was doing surprisingly well… you kind of enjoyed it, really. Being so stretched, enough to make you feel aroused and start the fire inside of your belly. Orgasm came at a rapid pace, and before you knew it, you were cumming around his cock. That would usually be enough to make him cum as well, and today wasn’t any different. He began to shoot spurts inside of you like normal, but this time he  _ didn’t _ stop. He just kept going, even while he was cumming.

It was kind of amazing, really, that his body was able to do that.

You were beginning to feel overstimulated, but you knew you couldn’t make him stop. If an orgasm didn’t phase him, then neither would your words, and you knew that. You took a couple of deep breaths and tried to keep control, but within minutes, you were a moaning puddle. You could barely keep up, and then  _ another _ orgasm washed over you.

You lost count after the third… you just remember him stopping very suddenly.

You felt him still, your body silently thanking him as your pussy pulsed and throbbed around him… he felt… thicker somehow, at the base. You looked down between you both and saw it was _ really swollen _ at the base of his cock.

You almost fainted.

He didn’t seem phased in the slightest as he began to lick your shoulder, where he had previously bitten you. It looked somewhat sore… how was he not uncomfortable with his dick being that swollen!?

You, on the other hand, felt like you had been bucked by a horse. Your body was throbbing and trying to calm back down after that experience. Everything just felt so sensitive and achy… you didn’t understand what was happening to him. He let out a soft sounding purr, kissing your neck and holding onto your hips. 

“...P-Pull out,” You asked, softly. Your throat was sore from screaming. You really wished you had some water.

He purred again, nuzzling into your neck. “i can’t.”

...That didn’t sound very good.

“...Why not?” You were sore… you couldn’t handle another round, not after  _ that. _

“knotted,” He replied, as if you were supposed to know what the fuck he was talking about. You could tell he was starting to come back to you, though… out of his drunken phase he seemed to be in this entire time.

“...It’s sore,” You complained, wrapping your arms around his neck again. You liked when he was himself. You loved him.

“sorry, angel. should come out soon enough,” He apologized, stroking your hair softly. It felt so nice… so comforting. You liked this much better. You felt much safer this way… the sexual thrill had been amazing, of course, but you did get overwhelmed pretty easily.

You stared at him for a moment, finding yourself getting a little lost in his eye sockets… they were beautiful and strange. You loved them very much. They were him… they were all him.

He leaned down and began to teeth at your neck, making you giggle just a bit. Your body was so incredibly sore, but that didn't stop your body from getting aroused again… Jesus, you could  _ not _ handle any more.

“...Stop it,” You said, but the giggle in your voice said otherwise.

“feels nice,” He protested… you knew he was right. It was very nice.

You sighed… your body still ached.

“sorry,” He started, pulling away from your neck… you knew you asked for it, but it still kind of bothered you when he did it, “it’s a primal thing. not sure why, but it just feels good to bite you for some reason.”

...Weird, but you didn’t mind.

You made a small humming noise and started to play with one of his hands. You always liked the anatomy. “Why are you acting like this, anyways?” 

You still didn’t understand.

“i’m in heat.”

“I still don’t know what that means.” You pouted. You thought you had already made that clear.

“...you really don’t know?” He looked surprised… maybe he thought you were pulling his femur earlier.

You shook your head in response.

He sighed. “how do i explain… do you know how dogs go into heat?”

You did, vaguely. You nodded and waited for him to continue.

“it’s like that. every few years i just… go into this craze and want to mate.”

“It’s… a breeding thing?”

“yeah. i want to fuck, i can’t think straight, i can’t hold my train of thought… all i can think about is making babies.”

You felt your face heat up at the thought of a baby with Sans… you weren’t opposed to it in the slightest, it was just something neither of you had ever talked about. It never came up in conversation on either side.

“...Babies?”

“mhm,” He confirmed, his expression softening as he began to stroke your hair again, “s’why i told you not to move. once you started moving, it triggered my instincts to hold you down and fuck you harder.”

You felt turned on by that thought.

“Do you… want babies?” You asked him… you knew his instincts and urges did, but you didn’t know if  _ he _ did.

“...with you? of course.”

You knew that must be different and hard for him. Down underground, he never would’ve been comfortable with having a child. But up on the surface… there was plenty of food and safety. It would be fine.

“You never told me you wanted kids.”

“i was nervous you’d think i was coming on too strong, kid.”

“There are casual ways to bring it up.” You knew you could have brought it up as well… that wasn’t all on him.

“just... a big scary skeletons wants to make babies and be permanent. i didn’t want to frighten you off with that.”

You smiled at him. “We are permanent.”

For the first time in a while, he gave a genuine smile back.

**Author's Note:**

> [My Tumblr.](https://www.tumblr.com/blog/skelesansation)
> 
>   
> [My Twitter.](https://twitter.com/skelesansation1)
> 
> [Discord server! Come chat. 18+.](https://discord.gg/f4xn8rB)


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